1. Field of the Invention* * All classes and subclasses listed in this section were obtained by searching the USPO web site. Most recent search: October 2002.
The present invention relates generally to a wireless communication channel and more particularly to a channel measurement technique that is employed concurrent with communications.
The invention combines a novel way to realize a vector network analyzer (class 324/subclass 615, 607, 606) and a communications link, the former operating concurrent with the latter, in such a way that information about the channel behavior is supplied directly and expeditiously to an adaptivity function associated with operating the communication link over a fading channel.
Examples of adaptivity aided by this invention are, adjusting the data rate, center frequency, type of modulation, bits per second, and forward error correcting coding.
Another adaptivity function is channel equalization (class 708/subclass 323). Prior art for equalization uses one or more of these methods: preambles, training sequences, embedded pilot tones, received and use of data-aided equalization (375/subclass 229, 232). With our invention, these equalization preambles, training sequences and pilot tones can be eliminated because the new vector network analyzer (VNA) produces the information needed to derive the correct equalizer, and does it better and faster than preambles, etc., with much less overhead.
Wireless unlicensed band communication and higher-powered licensed band communication are examples of fading channels (class 455/subclass 506). What is remarkable about our invention is that the fading channel being tested by the VNA is the entire propagation path, with the ends separated by hundreds of meters to tens of kilometers. Our invention is a breakthrough in the state of the art because it frees up the VNA from being merely a single piece of test equipment. Based on discoveries made concerning the elimination of artifacts, our invention allows a VNA to be distributed over very long propagation paths typical of wireless communications.
The resulting tight synergistic coupling of channel measurement with channel usage is not possible by any other means and can lead to the design of more efficient high speed channels for broadband communications.
Because the invention involves time interlacing of short measurement waveforms along with communications signals, this invention also has some resemblance to channel sounders (455/67.4) used both in pure research and in certain types of communications like meteor burst and HF communications. Taking meteor burst as a prior art example, channel sounding is fairly crude in the sense that it is a GO/NO-GO type of test, either there is a meteor trail or there isn't. Our invention sends precisely calibrated test signals time interlaced in an unobtrusive manner with communications. Some of the prior art channel sounders can measure amplitude versus frequency but cannot measure phase shift versus frequency as this invention can. Phase shift versus frequency is crucial to understanding the multipath behavior of the channel which, in turn, leads to a multiplicity of adaptivity options not available by other means.
The following Table 1 summarizes the field of the invention by reference to class and subclass categories as recently defined by the U.S. Patent Office.
TABLE 1Representative Prior Art: Categories Close to This InventionClassClass TitleSubclassSubclass Title375Pulse or Digital229EqualizersCommunications231Calibration of automaticequalizers224Testing232Equalizers with adjustabletaps240.02Adaptive coding depending onsignal324Electrical615Transfer Function TypeMeasuring DevicesCharacteristic607Including A/D Converters606Including Signal ComparisonCircuit455Telecommunications500Plural transmitters or receivers504Fading Compensation506Rayleigh or Multipath FadingDiversity Combining505Due to Weather515Control Channel Monitoring65Anti-Multipath67.4Using a Test Signal67.6Phase Measuring708Electrical Computers:300FiltersArithmetic Processing323Equalizersand Calculating
2. Description of Related Art
2.1. Previous Efforts at Overcoming Fading
Broadband wireless access networks under 5 GHz are being planned in the U.S. to operate in licensed bands that permit high power transmission, for example the MMDS, the ITFS, or the WCS bands, in the U.S. These bands are characterized by diffraction-mode propagation1 2 when the link is non-line of sight (NLOS). Diffraction is the phenomenon of waves bending around buildings and other obstacles such as foliage and rolling terrain. Attenuation also increases in a highly unpredictable manner in NLOS. In point-to-multipoint wireless networks there may be thousands of links which do not have a direct line-of-sight path between them and a central hub, each seeing a slightly different propagation path. Finally, whenever a subscriber has been off the air for some time it is necessary to recalibrate the link to account for minor changes such as weather conditions.
Under 5 GHz, where diffraction modes are significant3 4 5 6 7, use of the band in a purely line of sight (LOS) mode would be wasteful of the spectrum and would require a large amount of tower infrastructure. On the other hand, worst-case designs into the outer reaches of NLOS operation based on long-term statistical models of links, is also wasteful. Statistical models do not lead to results that scale into the network capacities needed for commercial viability of wireless Internet systems. The use of low power Industrial Scientific and Medicine (ISM) bands for broadband wireless access (BWA), while making the bands readily available without cost, would also require heavy infrastructure buildouts8.
Since most broadband applications today are two-way, the measurement should apply to both directions, adding to the complexity of improving performance over this type of link. The following is a summary of various prior art adaptivity techniques which have been contemplated for overcoming the effect of fading in wireless channels.
(a) Adaptation Through Use of Handshake Signals
Traditionally, wire line modems adapt to the physical medium by means of a start-up protocol which consists of trial messages, answering handshakes, and modulation fallback9 10. The two modems agree on a stable data rate and then begin information transfer. Wireless channels in diffraction mode change too fast for this method to ever reach equilibrium so another method is needed which works quickly and unambiguously to define the channel's capability.
(b) Handbook-Based Designs for Fading Channels
As a design approach, handbook-based channel models are a holdover from microwave radio relay fixed point-to-point links11. These are typically oversized by 20 dB to 30 dB to yield very high availability over the course of one year. The use of statistical channel models also finds use for mobile wireless telephony where doppler rates to a moving vehicle are high and predictability of the fine-grained channel structure in real time is nearly impossible12.
A number of models have been developed to enable engineering design to take place on a simulation basis and on a worst-case basis. Examples of these models are, Okamura, Longley-Rice, Egli, and Carey13, the Ricean K-Factor model14, and the Rayleigh fading model15. However, reliance on handbook-based channel models for guessing what broadband wireless channel characteristics are going to be, especially when those models are more appropriate to narrow band mobile environments16, is not a good idea.
(c) Embedded Pilot Tones Used to Adapt to Fading Channels
Yet another approach for training a receiver to deal with propagation conditions is the use of embedded pilot tones, specifically recommended in the literature for orthogonal frequency division multiplexing (OFDM)17, a modulation technique being considered by standards groups specifically for use with broadband wireless non-line-of-sight links. The literature has many examples of OFDM time-frequency occupancy plans in which certain slots are reserved for unmodulated pilot tones that step sequentially across the band18 19 20.
The embedded pilot tone approach suffers from the following problem: its determination of the channel is necessarily after-the-fact. What happens if the channel quality turns out to be too poor to support the data rate contained in that particular packet? Logically, it would be better to know what the channel's capability is before selecting the modulation, FEC, data rate, center frequency, and so on. Pilot tones also take up a significant percentage of the useful information-bearing capacity in the packet.
The invention described here overcomes another serious problem associated with embedded pilot tones, a problem that is not well treated in prior art systems. Since pilot tones are located at specific frequencies, the resultant channel estimate is at best a sampled version of the frequency response of the channel. If these samples are too widely spaced, the frequency transfer function needs to be interpolated. Sometimes this interpolation works, other times it may not work, for example if the delay spread in the channel is large, there could be a lot of selective fading. To make the problem worse, large delay spread is associated with high powered licensed bands where distances go up to many kilometers.
(d) Preambles Used as Channel Sounders for Adaptivity
Yet another adaptivity mechanism for use on fading channels, in particular to assist in finding an equalizer, is the use of a known training sequence as a preamble (or in some cases a “midamble”21) to each packet. This approach is typically used in Single Channel (SC) operation where selective fading causes intersymbol interference (ISI)22 23 24. Just as with embedded pilot tones, the channel adjustment mechanism called into play takes place after modulation and data rate have been selected, and so is of questionable value as a way to adapt to fading channel conditions.
(e) Diversity and Multiple Antenna Assemblies
The use of diversity techniques dates back many decades to line-of-sight radio relay systems, to HF communications, and to troposcatter. More recently diversity, and multiple input-multiple output (MIMO) antenna assemblies in particular, have been suggested for use in broadband wireless systems. The inventors believe that there is a lot of promise to this approach. Familiar with the extensive literature on the topic, the inventors set out to understand what needs to be done to implement diversity. We found out that having precise knowledge of the channel on each diversity path is just the first step in designing a diversity system, but by no means a guarantee of success.
A simple example is switch diversity versus equal gain combining diversity. With switch diversity, the strongest of several channels is selected and used. With equal gain combining diversity, all channels are used simultaneously by coherently summing together signals from several antennas.
Which is better, switch diversity or equal gain combining? Surprisingly, it turns out that even if knowledge of the diversity channels is perfect and both combining schemes are done correctly, than equal gain combining may or may not be better than switch diversity, depending on the signal to noise ratio of each diversity channel. This apparent paradox points out the criticality of first, knowing the channel conditions, and second, using that knowledge correctly. These two topics, measurement and adaptivity, are the subject of this invention.
The inventors are familiar with all of these techniques, (a) through (e), and have followed discussions in various standards setting bodies concerning the need for adaptivity in broadband fading channels25. We are convinced that it is necessary to decouple channel measurement from communication, as this invention does, in order to have the most freedom of adaptivity.
2.2 Related Art for Vector Network Analyzers
A laboratory measurement of a linear system's transfer function H(jω)\, say a filter characteristic or an amplifier pass band characteristic, may be accomplished with a tool variously called a network analyzer,26 vector voltmeter, and vector network analyzer (VNA). Before this invention, the VNA was limited in practical use to being a single piece of test equipment. \ Note, for simplicity in this application, the function H(jω) is not indexed by user, by inbound or outbound directions, or by the time variable, “t”, although it is likely to be a function of all three when applied to a propagation link.
Pahlavan and Levesque27 reported on using a conventional VNA for measuring the multipath delay spread characteristic in an indoor wireless environment. In their description, a conventional VNA was attached to the transmitter and receiver via long cables (tens of meters), and the VNA sweep probe was converted to a radio frequency signal and radiated into the indoor environment while the receiver's signal was brought back to the output port of the VNA after downconversion. The cited authors were then able to compute multipath characteristics in an indoor environment by appropriate signal processing of the measured H(jω) produced by their VNA.
Note that the use of a hardware connection to a single centralized device is out of the question over an active wireless link, but may be suitable for modeling and statistics gathering in a controlled indoor environment.
Getting around the problem of using cables to connect to a conventional VNA is not easy, but has been done before. In a paper by Baum28 a channel modeling experiment was described in which two rubidium frequency standards were used, one at each end of a wireless link in the San Francisco Bay area. They were able to find phase shift versus frequency for this link, thanks to the stability of the rubidium standards.
In another study done in 2000 at Virginia Tech by Hao Xu29 a conventional VNA connected by cables to the input and output of the link was suggested for doing short-range line-of-sight measurements of a 38 GHz wireless link multipath channel. In the same report, Xu goes on to describe a technique called a sliding correlation system (SCS) which he recommends for long distance propagation paths. As with the Baum experiment, Xu also uses rubidium frequency standards at each end of his SCS link. Briefly, his idea is to let the sliding correlator find correlation peaks due to multipath. In turn, the multipath peaks lead to a characterization, if desired, of amplitude versus frequency and phase shift versus frequency.
Using these two papers as a baseline, the inventors also searched U.S. patents which mention the words network analyzer30. It appears to us that that use of rubidium standards is the best and only prior art technique for realizing a distributed VNA. However, what is needed to achieve commercial viability and scaling to networks with thousands of users is a way to accomplish the equivalent measurement using inexpensive crystal oscillators, as well as a way to use the VNA concurrently with communications.
The inventors would now like to make clear their reasons for emphasizing the centrality of the transfer function H(jω) in wireless communications by citing some references and known facts.                The literature abundantly shows that optimal receiver processing for any communications link, not just fading links, depends on accurate knowledge of the channel transfer function31. For example, code word Euclidean distances32 with forward error correction (FEC) codes determine the ultimate bit-error-rate performance. In turn, the code's distance properties can be calculated once H(jω) is known. The amplitude part of H(jω) is particularly important in establishing a link budget and maximum possible bits per second, the phase shift part of H(jω) is particularly important for equalization and the successful use of coded phase modulation33         H(jω) exists as a physical reality and can be measured. The previously cited references by Pahlavan, Levesque, Baum, and Xu confirm the fact that H(jω) is central to understanding the ability of a wireless link to carry communications.        
Considering the difficulty evident in the literature in going from a single piece of test equipment in a laboratory to a distributed measurement of a propagation channel, it is worthwhile to examine carefully what makes these situations so profoundly different. The list which follows takes a step-by-step approach to demonstrating the artifacts which are introduced when it becomes necessary to have two widely separated end points, plus the use of inexpensive crystal oscillators at those end points. These are the difficulties the inventors had to overcome:                Separate local oscillator sources at transmit and receive ends will always inject some unknown phase noise process, referred to in this Application by φ(t), which directly cause errors in the phase angle part of the H(jω) measurement. In a single piece of test equipment one has the luxury of using the same oscillator at both ends of the device, so this problem doesn't exist. Note, that φ(t) is the end-to-end phase noise taking into account up-conversion and down-conversion to the frequency band being used.        Separate transmit and receive systems always have some initial uncertainty regarding the local oscillator long term stability settings, characterized as Δf/f which can throw off measurement of phase. Once again, the laboratory network analyzer using a single frequency source does not have this problem. The Baum and Xu experiments compensated for non-zero Δf/f by using rubidium frequency standards, carefully aligning the equipment to effectively yield Δf/f≈0 before and during each test.        Separate transmit and receive systems always have some initial uncertainty in synchronization. This Application refers to sync error as τε. The conventional VNA's probe signal in contrast to this invention is always in sync between input and output of the device under test.        Separate transmit and receive systems always have some initial uncertainty in the local oscillator up-conversion and down-conversion frequencies which causes an initial uncertainty in the received signal's center frequency, referred to here by fε.        All of these errors cause time-varying rotations in the received in-phase and quadrature components' complex number representation, masking the correct phase of H(jω) by large amounts in some cases, unless corrective action is taken. In this Application, the unwanted rotations are called artifacts, removal of which is described in DETAILED DESCRIPTION section 3.        The received measurement probe signal in a wireless transmission channel will be noisy, and may need to be filtered or processed before it can be useful.        Slow, stepped-frequency signals used in network analyzers are totally inappropriate for transmission over a communications network with thousands of subscribers, since this signal will interfere with data communications packets.        Subscriber equipment must be inexpensive, so the equipment-related errors described above, namely φ(t), τε, Δf/f, and fε, may be very significant especially soon after the subscriber first turns on the receiver. The challenge faced by the inventors was how eliminate these imperfections.        